1. Field of the Invention
The present invention relates to a method of producing a base compound for a liquid fluoroelastomer. In particular, the invention relates to a production method that yields a base compound for a liquid fluoroelastomer with minimal fluctuation in the viscosity between production lots.
2. Description of the Prior Art
Curable fluorine-containing elastomer compositions that utilize an addition reaction between an alkenyl group and a hydrosilyl group are already known, and compositions that have been imparted with self-adhesiveness by adding a third component composed of an organopolysiloxane having a hydrosilyl group and an epoxy group and/or trialkoxysilyl group have also been proposed (see Patent Document 1). Further, compositions in which a carboxylic acid anhydride is also added to improve the adhesiveness of the composition to polyphenylene sulfide resins and polyamide resins have also been proposed (see Patent Document 2). These compositions can be cured by heating for a short period of time, and the resulting cured products exhibit excellent solvent resistance, chemical resistance, heat resistance, low-temperature properties, low moisture permeability and electrical properties, and are therefore used in bonding applications in all manner of industrial fields that require these types of properties. The compositions are used particularly widely in the automotive industry, for sealing, coating and potting applications for electronic componentry.
Because the above compositions typically require a fluidity and a mechanical strength following curing that are appropriate for the intended application, a silica powder is often added to the composition. The amount of added silica is adjusted appropriately in accordance with the required performance. In order to ensure that the silica powder is dispersed uniformly through the composition, the silica powder is frequently added in the form of a liquid base compound that is prepared in advance by mixing the main component oil with the silica powder.
In an example of a conventional method that has been proposed for producing a liquid base compound, 25 to 60 parts by mass of a hydrophobic silica powder having a BET specific surface area of 50 to 400 m2/g is added in portions to 100 parts by mass of a linear fluoropolyether compound having at least two alkenyl groups within each molecule and having a perfluoroalkyl ether structure within the main chain, while the mixture is kneaded, either under conditions of heat and reduced pressure or under conditions of heat and pressure, and a post-dilution is then performed by adding more of the linear fluoropolyether compound to achieve a predetermined blend ratio between the components (see Patent Document 3). However, in this method of producing a base compound, the adsorbed water content of the silica powder added at the time of production changes depending on the environment (temperature and relative humidity) in which the silica is stored prior to use, and this variation in the adsorbed water content causes significant fluctuation in the viscosity of the resulting liquid base compound between production lots. This causes a problem in that the viscosity of fluoropolyether-based compositions produced using the liquid base compound as a raw material component also fluctuates significantly between production lots.
Methods that have been proposed to address this problem include a method in which a hydrophobically treated silica powder and water are added to the aforementioned linear fluoropolyether compound, and the mixture is kneaded without heating, and then further kneaded either under conditions of heat and reduced pressure or under conditions of heat and pressure, and a method in which the liquid base compound obtained using this production method is subjected to a post-dilution with the linear fluoropolyether compound in order to achieve a predetermined blend ratio between the components (see Patent Document 4). In these production methods, the viscosity of the obtained liquid base compound is stable between production lots, but a portion of the added water tends to remain adsorbed to the silica powder even after the heat treatment, and therefore when an aforementioned adhesive composition containing the liquid base compound is subjected to heat curing, foaming is frequently a problem. Accordingly, the development of a method of producing a liquid base compound that is capable of stabilizing the viscosity between production lots without requiring the addition of an unnecessary water component at the time of preparation has been keenly sought.
Patent Document 1: JP 3,239,717 B
Patent Document 2: JP 3,567,973 B
Patent Document 3: JP 2004-331903 A
Patent Document 4: JP 2008-69298 A